A class of optical circuits includes circuits known as planar lightwave circuits (PLCs). In such circuits, optical signals received from input terminals are selected, redirected and transmitted to output terminals. Often, the redirection is performed using an array of switches.
In fully optical circuits, optical signals are carried along waveguides. Waveguides are typically formed as a doped core region situated within a substrate. The doped core region and the surrounding substrate generally have different refractive indexes. An optical signal is guided through the substrate along the waveguide. Perturbations may be formed within or along the waveguide to perform switching operations on the optical signals traveling in the waveguide, for example. Typically, the waveguides are laid out as a grid having intersections, or cross points, and the perturbations are formed at the intersections.
A perturbation can be a liquid-filled trench used as an optical switch, a doped portion of the waveguide, or some other structure or material having a refractive index different than the refractive index of the waveguide. The refractive index of a switching perturbation is capable of being changed between a number of possible states, for instance, between two levels. One of the levels causes the optical signals to pass through the perturbation without changing direction, and the other level causes the optical signals to change direction and pass into an intersecting waveguide. In the case of a liquid-filled trench, the liquid may be moved aside in some manner to leave a gaseous phase at the intersection; for example, a bubble may be formed in the trench by heating the liquid. In general, the perturbation defines a three-dimensional index of refraction distribution, positioned at an intersection of two waveguides; changing the value of the refractive index performs a switching operation.
Prior Art FIG. 1 illustrates an exemplary switching device 10 that is able to switch M input optical paths (designated Input-111, Input-213, . . . , Input-M 15) into N output optical paths (designated Output-117, Output-219, . . . , Output-N 21). The input and output paths can be waveguide segments; however, although described herein as such, in general the optical paths can be any optical path capable of conducting an optical signal. Device 10 includes optical switches (designated Si,j, where i is the input row and j is the output column) at each waveguide intersection. Prior Art FIG. 1 illustrates two of the possible optical paths through device 10. The first path, designated by vectors 12a and 12b, shows an input signal Input-111 directed via switch S1,2 to the output terminal Output-219. The second path, designated by vectors 14a and 14b, shows an input signal Input-2 directed via switch S2,N to the output terminal Output-N 21.
Prior Art FIG. 2 illustrates a view of section 18 of device 10. Section 18 includes a fluid optical switch S2,2 having a trench 22. Switch S2,2 is defined by trench 22 formed between a break in waveguide 24 and other layers of device 10 including, without limitation, cladding layers 26 and heating circuit layer 28, including heating element 29. The layers of device 10 are typically built on a substrate 30. Such switches are known in the art.
A variation in optical power within an optical circuit may cause deleterious effects, including detector saturation and inter-channel cross-talk, that can lead to transmission errors. Because power fluctuations occur dynamically within an optical circuit, it is of interest to have a means of adjustment. This is the function of variable optical attenuators (VOAs).
The VOAs of the prior art each have their disadvantages. Prior art VOAs may be too large, too expensive, or require too much power. Prior art VOAs may also have a slow response or a high insertion loss. In addition, prior art VOAs may not be readily integrated with optical circuits and switches such as those described above. Accordingly, there is a continuing need for VOAs that provide an improvement over the disadvantages of the prior art.